There has been generally used a transformer in which large current of 100 A or more flows through a low-voltage coil on a secondary side, for a power transformer or a DC-DC converter used for an electric car, a larger server or the like. In such a transformer, a flat metal plate having a small resistance is normally used for the low-voltage coil in order to accommodate increased current.
In the case where the low-voltage coil is formed of the flat metal plate, the low-voltage coil on the secondary side and a high-voltage coil on a primary side cannot have a layer winding structure, and thus there has been adopted a structure in which the low-voltage coil and the high-voltage coil are separately disposed. However, some of such structures have a problem of difficulty to obtain desired characteristics due to insufficient coupling.
Therefore, as illustrated in FIGS. 5 to 8, there has been conventionally used a transformer 10 in which a high-voltage coil 20 with a wire t wound around a winding drum 40a of a bobbin 40, and a low-voltage coil 30 formed of a flat plate member having an open ring shape, are alternately laminated in their axial directions, and cover members 50 each having a ring plate shape are coaxially laminated on outsides of the laminate in its axial direction. Note that reference character 40b denotes a flange of the bobbin 40. The reference numeral 60 denotes each of a pair of E ferrite cores surrounding the low-voltage coils 30 and the high-voltage coil 20 to form a figure-8 shaped closed magnetic path. The cover member 50 is an insulation member and interposed between the E-shaped ferrite core 60 and the low-voltage coil 30.
In the conventional transformer 10 configured as described above, the high-voltage coil 20 is sandwiched between the two low-voltage coils 30, so that coupling can be enhanced, thereby obtaining excellent characteristics.
The power transformer 10 having the above structure is also disclosed in Patent Literature 1, for example, recited below.